The contractile strength of heart muscle is strongly dependent on the rate of beating, and on the pattern of previous contractions over a period of many seconds. The activator calcium ions, which are responsible for the magnitude of contraction triggered by each action potential, are released from two morphologically and functionally different cellular stores. The characteristics of these stores and the cellular movements of activator calcium were studied by means of simultaneous microelectrode and tension recordings, and measurements of extracellular Ca('++) concentration with Ca('++) -selective electrodes. These electrodes were made from one of the neutral ligands developed by Simon and colleagues (ETH 1001), with the electrode tip diameter 2 to 5 micrometers. The time constant of the electrode-recording system was less than 1 second.Changes in calcium activity were measured during steady states at several frequencies and during rest intervals, with the following results. During rest intervals, extracellular Ca('++) concentration changed in two phases: an initial decline for a few seconds was followed by a slow increase through rest intervals up to more than 60 second duration. Thus, it is concluded that most of activator calcium, which leaves the intracellular stores (T-sites) after a few seconds, is transported across the sarcolemma into the extracellular space. Experiments with ryanodine show a larger Ca('++) efflux and indicate that under normal conditions about one third of this released calcium is bound at membrane-sites (M-sites). The Ca('++) transport across the sarcolemma is a Na('+)-dependent process. Vanadate inhibits Ca('++) efflux and potentiates M-site Ca('++) for increased contractile strength at low frequencies. From extracellular calcium measurements during steady-state contractions, it was found that a smaller amount of Ca('++) is exchanged per beat between the cells and extracellular space at 2 contractions per second than at 1 cps. This confirms that the stronger contraction at higher frequency is due to an increase in Ca('++) made available from intracellular stores.